1. Field of the Invention
The present invention relates to a wheel slip angle detecting system for a vehicle, designed to determine a slip angle of each of wheels (an angle formed by the direction of movement of the wheel with respect to the orientation of the wheel) to grasp a road surface contact situation of each wheel, and more specifically, a lateral force itself of each wheel, in order to increase the accuracy in the control of the motion, e.g., of the turning movement of the vehicle.
2. Description of the Related Art
For example, there is conventionally known a technique of detecting a slip angle of each of wheels in the control of the motion of a vehicle (e.g., in the traction control, the antilock brake control and the like), such as the one disclosed in Japanese Patent Application Laid-Open No. 9-142280.
In Japanese Patent Application Laid-Open No. 9-142280, to determine the slip angle of the wheel, a lateral slip angle of a vehicle body (an angle formed by the direction of movement of the vehicle with respect to the orientation of the vehicle) must be calculated. It is a conventional practice to calculate a lateral slip angle .beta. of a vehicle body by integrating a differentiated value of lateral slip angle determined according to a differential equation based on a linear two-wheel vehicle's motion model. More specifically, a differentiated value d.beta./dt of lateral slip angle .beta. is calculated according to the differential equation {d.beta./dt=(AY/V)-.gamma.}, and a lateral slip angle .beta. is determined by integrating the differentiated value d.beta./dt, wherein AY represents a lateral acceleration of a vehicle body; .gamma. represents a yaw rate; and V represents a vehicle speed.
However, when the vehicle is in a state traveling straightforwardly at a low speed, detected values of the lateral acceleration AY, yaw rate .gamma. and vehicle speed V are small. In a system designed so that a lateral slip angle .beta. is determined by integrating such detected values, a sensor noise (a mounting error noise) of detector means for detecting a lateral acceleration AY, a yaw rate .gamma. and a vehicle speed V and a travel noise are accumulated in the course of integral calculation, resulting in poor lateral slip angle calculation accuracy and in reduced wheel slip angle detecting accuracy.